Method and device for heating foils and arrangement for measuring foil temperatures

ABSTRACT

The invention concerns an improved method of heating or tempering foils and an associated device including an arrangement for measuring the temperature, based on the following features: a separate arrangement is provided for heating the foil edges; the foil edge heating arrangement comprises an infrared radiator device ( 21 ) and a hot air heating device ( 29 ); and the infrared radiator device ( 21 ) and the hot air heating device ( 29 ) can be controlled or set at different heating output values.

BACKGROUND OF THE INVENTION

The invention relates to a method and an apparatus for film heating andto an associated measuring device for measuring the film temperature.The invention thus refers to thermoplastic films which are preferablyproduced, for example, on the basis of polyester, polypropylene orpolyamide, the preferred materials, namely polyester, polypropylene or,for example, polyamide, not having to be in pure form, but the filmsalso being capable of being produced, using modifications of thesematerials and/or, moreover, using further admixtures and/or additives.

Devices for the heating and thermal control of films, particularly alsoin the case of simultaneous stretching processes, are known.

Thus, for example, U.S. Pat. No. 5,429,785 describes a simultaneousstretching plant having a plurality of preheating and intermediateheating devices. These heating devices consist of hot-air heatingdevices or of infrared radiators. However, a combination of both mayalso be used for the heating and thermal control of the film. In themethod, known from the abovementioned U.S. patent specification, for theproduction of ultrathin films with a final thickness of less than 2.5μm, the abovementioned heating devices can be provided at a plurality oflocations in the stretching operation: they are in each case arrangedtransversely to the drawoff direction of the film web so as to cover theentire film width.

U.S. Pat. No. 5,071,601 discloses a method of producing a thermoplasticfilm. Here, the film is guided via a plurality of rollers slightlytapering conically, in order, via these, ultimately to produce a curvedfinal film which is used, for example, as an intermediate film formultilayered glazing for motor vehicles. The heating device required forthe process of producing the plastic film consists of a plurality ofheating zones of an infrared heating device which are arranged, offsetrelative to one another, transversely to the drawoff path of the plasticfilm, the heating zones generating increasingly higher temperatures fromone edge region of the film to the opposite edge region. This isnecessary in order to achieve the desired curved film web profilementioned. In this case, however, due to the different materialstretching, the thickness of the film web has different values, althoughthis is of only minor importance in the specific use and specificservice of the plastic film web for the production of motor vehicleglazing.

It was then shown, however, that, in the simultaneous stretching ofplastic films, although relatively uniform stretching is possible in themiddle region of the film web, problems still arise, above all, in thefilm edge region. This is because, on the one hand, the film edge regionalways has a greater material thickness than the remaining middle filmweb portion. However, the stretching conditions are also impaired due tothe fact that poorer heat transmission can be detected at the edge ofthe film web, this also being caused, inter alia, by the tenterhooksengaging on the film edge.

The object of the present invention is, therefore, to provide a method,as well as an apparatus for carrying out the method, and a suitable filmtemperature measuring device, which makes it possible to produce plasticfilm webs of improved quality in a stretching process.

Applicants have found that, in the stretching of films in longitudinal,transverse or simultaneous stretching processes, edges which arethicker, as compared with the middle of the film, must be thermallycontrolled separately before and during the stretching process, since,because of the greater thickness and the poorer heat transmission at theedge, the desired temperature cannot be achieved solely by blowing withstandard nozzles. Another reason for the necessary different thermalcontrol of the film edge and of the middle region of the film web isthat the stretching behavior is different in each case. Thus,specifically, the film edge gripped by the tenterhooks is stretchedessentially longitudinally, whereas the remaining film web material isstretched biaxially.

Furthermore, particularly in the case of simultaneous stretching, thefilm edge performs the essential function of the introduction anddistribution of force. Since the rigidity of the edge can be influencedwithin wide ranges by means of the temperature, a defined setting andcontrol of the edge strip temperature assumes appreciable importance.

In the previous stretching plants in general and the simultaneousstretching plants in particular, the film edge has not been thermallycontrolled separately. Moreover, hitherto, there have also been nosolutions for subjecting this edge to special heating in a controlledmanner. At the same time, the partitioning off of the film edge by thetransport system, that is, above all, the tenterhooks engaging on thefilm edge at discrete intervals, presents a particular problem, sincedirectional heating of the film edge is thereby impaired even further.

Admittedly, WO 94/047 WO has disclosed an apparatus using injectornozzles to blow textile cloth webs transported on, spread out, saidapparatus comprising a blowing or nozzle box facing the top side and theunderside of the cloth web. This textile drier is arranged transverselyto the cross web. If special edge drying is desired, nozzles may also bedirected only onto the edge regions of the cloth web or be subjectedonly there to treatment gas. To that extent, however, this is anongeneric prior art, since the subject of the present applicationrelates to an apparatus for the thermal control of plastic film websduring simultaneous stretching processes, and, in this case, specifictemperature distributions must be achieved in the plastic material crosssection.

Furthermore, DE 25 42 507 A1 has disclosed an apparatus for the zonalregulation of the thickness of a stretchable thermoplastic film web. Inthis known apparatus, separate film edge heating is not provided, but itis merely proposed to arrange above and, if appropriate, also below thefilm web parallelepipedic air wells which each have an inlet and anoutlet for the hot air capable of being supplied, the throughflowquantity of the hot air supplied being variable in the individual wellsby means of blind-like individually moveable slats. This known apparatusdoes not allow any actual separate film edge heating, however, since hotair having one temperature level can be made available only uniformlyfor the thermal treatment of the entire plastic film web. Thetemperature cannot be regulated zonally, only the hot air quantity.Moreover, heating is to take place solely by means of hot air, withoutthe recognition that optimal film edge heating, and thereforetemperature regulation for the film edge, can be achieved precisely bycombined heat treatment by means of hot air and infrared irradiation.

BRIEF DESCRIPTION OF THE INVENTION

The present invention affords numerous advantages. By virtue of theinventive edge strip heating for simultaneous stretching plants, thequality of the plastic film webs to be produced is markedly improved, ascompared with conventional films produced. The material edges of aplastic film web, which are thicker as a consequence of production, cannow be thermally controlled and heated directionally, in such a waythat, in a simultaneous stretching process, this edge region of aplastic film web can also be optimally stretched. In order to achieve,overall, an optimal stretching condition for the film in terms of therunning stability of the latter, on the one hand, and of theintroduction of force from the film edge to the film web to bestretched, on the other hand, it is necessary for the film edge and therest of the film material to be heated to the same temperature, despitethe different thickness. By means of the inventively combined separatethermal control operation for the film edge by means of hot air andinfrared radiation, almost any desired temperature profile can be set,and implemented, over the width of the film edge.

The use of infrared radiation for the heating and thermal control of thefilm edge is particularly suitable. This is because the film materialcan absorb the energy or output of the infrared irradiation over asubstantially shorter distance, and therefore within a substantiallyshorter time, than hot air on account of the higher heat transmission.Particularly because short-wave rays (1.1. μm) are used, the radiationpenetrates more deeply into the film edge material. At the same time, ahigh output can be generated in a small space and be introduced into thefilm. Preferably concentrated radiation results not only in a highoutput, but also in heating over an exact area. In this case, however,the surface of the film could be damaged by an excessive introduction ofheat, at the same time the underside of the film possibly still beingbelow the temperature ranges to be achieved per se. The solutiontherefore lies in the simultaneous concentrated action of air at highvelocity on the film edge. The hot air is set to a specific desiredtemperature, in such a way that the process is controlled via this sothat rapid heating, which is uniform over the film thickness, isachieved (equalization of the heat introduced as a result of theinfrared radiation). Particularly for special edge geometries with athickness profile decreasing toward the middle of the film, theadditional air is helpful and is important in order to prevent thethinner regions of the edge profile from being overheated by theinfrared radiator and destroyed.

The factor essential for the introduction of energy is due to theinfrared radiator. At the same time, the fact that the thicker film edgehas a higher absorption behavior, as compared with the thinner filmmaterial portion adjoining it, has a positive effect. The absorptionbehavior is therefore dependent on the film thickness. This can beutilized particularly effectively in so-called bright radiators(radiator temperature above 2000° C.). This means, for the heatingoperation, that the thinner film material transmits more radiation thanthe comparatively thicker film edge, so that the thinner film materialportions adjacent to the edge therefore cannot be overheated, whichwould lead to tears during stretching.

However, a slightly higher temperature of the film cannot be avoidedwhen the adjustable lateral blowing nozzles, preferably providedspecially for film edge heating, are operated at the so-called processtemperature. Thus, whereas, for example, the plastic film (with theexception of its edge regions) can be subjected overall to hot air inthe corresponding heating zones at a temperature of, for example, 93° C.(=process temperature), a process temperature set slightly lower for thehot air is preferably used in the edge region. However, due to thecombination with the infrared radiator, a reduction in the airtemperature for the lateral blowing nozzles to, for example, 90° C. thenmakes it possible for the temperature between the edge and the remainingfilm material to be set very accurately to an almost constant desiredtemperature level.

The inventive advantages of almost entirely constant thermal control ofa film over the entire width are afforded in a combination of infraredradiation and convection (blowing with hot air), even when this combinedthermal control operation is carried out from only one side (forexample, the top side of the film). Even here, the temperature profileis actually set equally constantly and uniformly over the entire filmthickness. In this case, maximum temperature deviations of, for example,2° K. or 1° K. over the entire film edge thickness are possible.

A measuring device according to the invention for the appropriatesetting of the desired temperature profile even in the film edge regionis distinguished by the use of a pyrometer, the setting time of which issuch that the interchange between tenterhooks moving past and film doesnot lead to a signal fluctuation. If, furthermore, measurement,determination or presetting of the temperature of the tenterhook istaken into account, the edge strip temperature can ultimately beascertained comparatively accurately from these data and may then serveagain as an initial control variable for activating the heating device.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained below by means of exemplary embodiments withreference to several drawings in which in particular:

FIG. 1: shows a diagrammatic top view of a simultaneous stretchingplant;

FIG. 2: shows a diagrammatic cross-sectional illustration through atentered web at a location adjacent to a film edge;

FIG. 3a: shows a graph to make clear the temperature profile in the filmedge;

FIG. 3b: shows a diagrammatic cross section through a plastic film inthe edge region;

FIG. 4: shows a diagrammatic illustration for measuring the temperaturein the film edge region;

FIG. 5: shows a diagrammatic optical reproduction for determining thefilm temperature in the edge region; and

FIGS. 6.1 to 6.4 show various temperature profiles in the film, and inthe film edge.

DETAILED DESCRIPTION OF THE INVENTION

The diagrammatic top view according to FIG. 1 shows a simultaneousstretching plant for producing plastic film webs, in which, as is known,a plastic web of comparatively small width, coming from an extruder notillustrated in any more detail, is gripped at its two edges by means ofso-called tenterhooks, via an intermediate cooling drum arrangementlikewise not illustrated, in the entry region of the simultaneousstretching plant. In this case, the tenterhooks, or tenterhook carriage1, shown in FIG. 2 are moved on the two laterally rotating tracks 3, forexample by means of a linear motor drive. At the same time, the plasticweb 9 is appropriately thermally controlled or heated, for example in aso-called infeed zone 4, a subsequent preheating zone 5, with theplastic film web width still remaining the same, and a subsequentsimultaneous stretching zone 7 as well as a further subsequentrestretching zone 8, corresponding film edge heating devices beingrepresented in FIG. 1 by the reference symbol 6. Furthermore, theso-called thermally controlled zone may also be followed by anotherrestretching zone and a so-called relaxation and/or cooling zone, inwhich zones the plastic film width can be kept essentially constant orbe set slightly narrower, as compared with the maximum plastic film webwidth at the end of the simultaneous stretching zone 7. At the end ofthe stretching plant illustrated in FIG. 1, the plastic film web 9 isthen released by the opening of the tenterhooks and is conveyed furthervia various draw rollers.

FIG. 2 shows, for example, a diagrammatic cross section along the lineII—II in FIG. 1, which illustrates diagrammatically that tenterhooks, ortenterhook carriages 1, are driven, for example via a linear motordrive, on a rotary track 3 of rectangular cross section, the tenterhookcarriages being capable of running, and being held, via a multiplicityof running rollers 11 on the opposite horizontal and vertical runningsurfaces 13 on the rails 3. The magnet coils 10 for the linear motordrive may, for example, be arranged in each case at a fixed locationalong the rail track 3, plates with permanent magnets 12 being designedon the tenterhooks, and being separated from said rail track by a smallclearance.

The film edge 9′ of the plastic film web 9 is clamped on the tenterhooktable 15 in a known way, specifically by means of a pivotably heldtenterhook lever 17 which is brought into the closing position and whichis illustrated merely by dashed and dotted lines in FIG. 2.

In the exemplary embodiment shown, infrared radiators 21 areaccommodated above the film edge 9′ in the cross-sectional illustrationand, at the same time, also above the upper end of the pivotabletenterhook levers 17 themselves, in each case in the heating zones ofthe plastic film web. Here, the radiator source 23 is equipped on theside facing away from the film, that is to say overhead in the exemplaryembodiment shown, with a reflector 25 which is of concave design, thatis to say is shaped with a preferably parabola-like cross section, as aresult of which concentration and focusing of the infrared rays in thedirection of the film edge 9′ can be achieved.

To protect the reflector surface and the radiator source itself, theunderside of the infrared radiator 21 thus formed may also be coveredwith a protective plate, for example a glass plate 27. For betterfocusing, lenses 27 or diaphragms may also be used. Moreover, mirrorreflectors may be arranged on the underside of the film edge.

Furthermore, a hot-air heating or thermal control device is alsoprovided for heating the film edge 9′. This device comprises a hot-airsupply duct 31 which is laid above the respective rail 3 and merges intoa vertical hot-air duct 33 and of which the slit nozzle 35 runningparallel to the tenterhook rail track 3 in the heating region islikewise aligned with the film edge 9′.

FIG. 3b reproduces diagrammatically, in an enlarged cross-sectionalillustration, a different film thickness profile, above all in the edgeregion, which, according to FIGS. 3a and 3 b, reaches about as far asF_(RB). Here, the distance A from the outermost film edge F_(R) isreproduced on the X-axis. It is clear from this that the film thicknesswhich is produced, for example, in a final plastic film, and which maymove within a μ-range, is thicker by a multiple in the film edge region9′ in and prior to the stretching zone.

FIG. 3a, in this case, reproduces the achievable temperature profile T,also in the film edge region, dots indicating which temperature profileT_(A) would be achievable if only a hot-air heating device were used,and dashes illustrating which temperature profile TB would be achievableif only an infrared radiator were used.

The unbroken line T_(C) reproduces the temperature profile actuallycapable of being set when both an infrared radiator and convectionheating, using a hot-air heating device, are employed. In this case, hotair, to which the film edge is subjected, is set at a temperatureslightly lower than the actual process temperature. By the processtemperature of, for example, 93° C. is meant that temperature at whichthe plastic film web is to be set overall, particularly during thesimultaneous stretching operation. The temperature selected slightlylower for the action of hot air compensates for the infrared irradiationsetting, per se, a temperature in the film edge region which is somewhatabove the process temperature T_(P).

At the same time, by slight variations, for example in the temperatureof the hot air, and by slight changes in the energy emitted by theinfrared radiator, the temperature profile range in the film edge regioncan be set differently, depending on the desired conditions, for examplein such a way that the desired temperature profile T_(C) in the filmedge region moves within the temperature band ΔT_(C1) or within thetemperature range ΔT_(C2). That is to say, the temperature range ΔT maybe set to rise slightly toward the outermost film edge F_(R), forexample to remain constant horizontally or even to fall slightly. Slighttemperature fluctuations within the temperature bands ΔT drawn in FIG.3a are insignificant, since these fluctuations are only extremely smalland do not have any adverse effects.

The temperature ratios which are set have been simulated and areillustrated with reference to FIGS. 6.1 to 6.4.

Here, the graph according to FIG. 6.1 reproduces the ratios when thefilm enters the heating zone. If the film has a starting temperature ofapproximately 80° C., for example before it reaches the heating device,the temperature rises in short time intervals of, for example 0.1seconds. In other words, the temperature rises to a value of about 95°C. (the temperature values are indicated on the X-axis), thistemperature value being set virtually over the entire thickness of thefilm. In this case, FIG. 6.1 reproduces the film thickness in crosssection, the film top side being illustrated at the top with 0.0 μm andthe film underside at the bottom with 0.012 μm. This film thicknessoccurs next to the thickened edge region at the transition to the thinfilm cross section (for example, at the location F_(RB) in FIG. 3b)

FIG. 6.2 reproduces the temperature profile in the thickened film edge(here too, the film top side being illustrated at the top with 0.0 μmand the film underside at the bottom with 0.25 μm). A slightly highertemperature is set, here, on the film top side, whereas the temperatureis below 93° C. on the film underside.

The ratios according to FIGS. 6.1 and 6.2 occur when, in addition toinfrared irradiation, air with a process temperature of, for example,93° C. is blown onto the film edge.

If, as illustrated in FIGS. 6.3 and 6.4, the film edge is blown at aprocess temperature of, for example, 90° C., it is shown that thedesired process temperature of 93° C. is set virtually constantly, overthe entire film thickness, both on the thickened portion of the filmedge (FIG. 6.4) and at the region of transition to the thinner filmportion in the middle film region.

Corresponding heating devices, known per se, for heating the remainingmiddle film material portion, in particular using hot air, are notillustrated in any more detail in the figures relating to the exemplaryembodiment shown.

The wavelength of the infrared radiator may be selected accordinglywithin wide ranges. The advantage of short-wave radiators with awavelength of about 1.1 μm is that they allow energy to be introducedinto deeper film edge layers, since air heats, above all, the surface ofthe film edge. By virtue of the concentrated radiation mentioned and thefocusing brought about thereby, the edge regions can be irradiated andheated directionally with a predetermined energy cross section. Ifrequired, the infrared radiators, may, furthermore, also be cooled bymeans of integrated water cooling, in order to generate and radiate ahigh output in a small space (an integrated cooling line in the infraredradiator is designated by 37 in the drawing).

Reference is made below to FIGS. 4 and 5, by means of which atemperature measuring device is explained, in order, on the basis of thefilm edge temperature determined, to activate and operate directionallythe film edge heating devices explained above.

In order to set and control the desired optimized temperature profileeven in the edge region, it is necessary to measure the edge temperatureof the film. However, the film edges are held at particular intervals bythe tenterhooks mentioned, depending on the stretching ratio, thetemperature between these tenterhooks being relevant to the process ofsimultaneous stretching, although the tenterhook temperature itself maycause the measurement result to be falsified.

Since accurate measurement of the temperature of the film edge while theplant is in operation, that is to say during the permanent rotation ofthe tenterhooks, is therefore not directly possible, contactlessmeasurement by means of pyrometers is proposed. In this case, however,contactless measurement by means of a pyrometer does not yet give thedesired sensing rate with the necessary narrow band width of thedetector or does not supply the desired accuracy because the temperatureis recorded too slowly. Only a falsified mixed signal is thereforeultimately measured, which includes the temperature of the tenterhooks 1moving past and the temperature of the film edge 9′ in the interval a(FIG. 4) remaining between two adjacent tenterhooks 1.

Film edge temperature measurement and a film edge temperature measuringarrangement, using at least two pyrometer arrangements 41, 43, aretherefore proposed.

By means of a first pyrometer arrangement 41, only the tenterhooktemperature is measured by means of a wide-band slow pyrometer (that isto say with a long response time), in such a way that at least onetenterhook 1 is continually detected. According to the diagrammaticillustration shown in FIG. 4, this can be ensured by orienting thedetection direction 45 of the first pyrometer arrangement 41 with atangential component to the film edge 9′ and, consequently, to therespective rail portion 3, on which the tenterhooks 1 are moved along.

Via the second pyrometer arrangement 43, a mixed temperature is measuredby means of a narrow-band pyrometer which is designed for the respectivetype of film and has a long response time and the setting time of whichis such that the interchange between tenterhooks 1 moving past and thefilm 9 or film edge 9′ does not lead to signal fluctuation. In theexemplary embodiment shown, the second pyrometer arrangement 43 isoriented at right angles to the film edge 9′, that is to say, as a rule,essentially transversely or at right angles to the film web plane, thedetection direction 47 being aligned with the film edge 9′ moved past ineach case, and the tenterhooks 1 also being moved through here in theinduction region.

A signal S_(C) for the tenterhook temperature (measured by the firstpyrometer arrangement 41) and a mixed signal S_(F+C) for the mixedtemperature consisting of the film temperature and of the tenterhooktemperature are reproduced diagrammatically in FIG. 5.

By means of an electronic central control and evaluation device, notillustrated in any more detail, particularly using a microprocessorcircuit arrangement, the actual film temperature S_(F) can be determinedcontinuously and contactlessly during production, as illustratedgraphically in FIG. 5 (the film edge temperature always falling to thetenterhook temperature in the region of the tenterhook and risingrapidly to the actual film temperature S_(F) again in between). In FIG.5, the signal magnitudes S determined are plotted against the time axist and the formulas for the dependence of the film temperature T_(F) arereproduced.

In this case, furthermore, the actual film temperature or film edgetemperature can be determined in the evaluation and control system,taking into account the current geometric ratios (tenterhook size,tenterhook interval a, tenterhook sequence b, etc.) and a correctingfactor, and can be converted to the temperature of the edge strip.

Finally, calibration of the pyrometers 41, 43 is also possible. When thesimultaneous stretching plant is running without film, a calibratingplate 51, illustrated in FIG. 4, having an automatic calibratingsequence can be heated, a measuring sensor 51′ being integrated in theplate and the measured temperature being monitored. In this case, thecalibrating plate 51 is located in the detector range 47 of the secondpyrometer arrangement 43. At the same time, the temperature measured bythe pyrometer 43 is compared with the temperature measured by themeasuring sensor 51′ assigned to the calibrating plate 51, and acorrecting factor correspondingly included later in the evaluation isdetermined.

Via the edge strip temperature measurement calibrated in this way, theinfrared radiators and/or the hot-air heating device can then becontrolled.

The accuracy of the measurement may be increased by providing a definedbackground in the form of a black radiator which, during measurement,leads to a defined background with a constant temperature, that is tosay defines the transmitted background radiation, and, on the otherhand, can be utilized for calibrating the system during idling phases ofthe machine (black plates).

The combination of the directional influence exerted via radiation andhot air with the edge strip temperature measurement may be utilized as aclosed control loop for an exact setting of the edge strip temperature.

The edge strip heating explained for simultaneous stretching plants canbe installed, and used, on different sections of the plant, thus, forexample, in the infeed zone 4, in the preheating zone 5, in thesimultaneous stretching zone 7, but also in the restretching zone 8, asillustrated diagrammatically in FIG. 1 by the reference symbol 6.

The exemplary embodiments have been explained in terms of a situationwhere infrared irradiation and hot-air action are in each case carriedout simultaneously. That is to say insofar as a double heating device,specifically an infrared radiator and a hot-air heating device orhot-air discharge nozzles, is provided in each case on correspondingheating sections 6. It is perfectly possible, however, to envisageinstances of use, in which the heating of the film edge by means ofinfrared radiators or by means of hot air does not always or exclusivelyhave to take place simultaneously, that is to say simultaneously withrespect to a specific portion of the film edge. Moreover, there may,instead, be provision, during the forward movement of the film web to betreated, for providing, on the plant, additional plant sections, inwhich only infrared heating or only hot-air action is additionallycarried out. It may therefore also be envisaged that the double heatingexplained takes place in an at least partially staggered manner in thelongitudinal direction of the plant, so that the plant zone for infraredheating and the plant zone for hot-air action overlap only in portions,so that, in these overlapping portions, said infrared irradiation andhot-air action take place simultaneously and, in the portions which donot overlap, only infrared irradiation or only hot-air action takesplace.

Even though, with regard to the hot-air heating device, the exemplaryembodiment has been explained in terms of the situation where heated airis supplied to the film web or the film edge, the expression “air”refers to any suitable gas mixture which may be used for this purpose.

What is claimed is:
 1. A method of heating and maintaining thermalcontrol of a plastic film in a stretching process, wherein the plasticfilm is a web having a main central portion and two parallel edges, themethod comprising: (a) heating the entire plastic film web as a wholewith heated air, infrared irradiation or both, and (b) heating andthermally controlling the film edges by applying in combination hot airand infrared irradiation in a controlled manner thereby setting oractivating the film edges and thereby reducing a temperature deviationof the film edges from a desired process temperature.
 2. The method ofclaim 1, wherein in step (b) the hot air is applied to the film edges ata temperature below a desired process temperature or a desired filmtemperature.
 3. The method of claim 1, wherein the infrared irradiationapplied is focused on the film edge.
 4. The method of claim 1, whereinthe infrared irradiation and hot air are applied and controlled inaccordance with the temperature of the film edge.
 5. The method of claim4, wherein the film edge temperature is determined by indirectmeasurement from a plurality of locations on the film edge and themeasured temperature values are compared.
 6. The method of claim 1,including the additional step of: (c) providing additional heat to theentire film web as a whole by applying hot air, infrared irradiation orboth.
 7. Apparatus for the heating or thermal control of a plastic filmhaving a central portion and two parallel edges in a plastic filmstretching processes, said apparatus comprising: a hot-air heatingdevice for subjecting the plastic film to hot air, or an infraredradiator device for irradiating the plastic film by means of IR rays orboth hot air and IR rays, a separate device for heating the film edgewhich comprises in combination an infrared radiator device and a hot-airheating device, wherein the infrared radiator device and the hot-airheating device can be activated or set to independent heatingtemperatures.
 8. The apparatus according to claim 7, wherein theinfrared radiator device includes a focusing device in the form of ahollow reflector or parabolic mirror.
 9. The apparatus according toclaim 8, wherein the infrared radiator is provided with an integratedcooling device.
 10. The apparatus according to claim 7, wherein theplastic film is supported and transferred on a series of leveredtenterhooks during heating and stretching operations.
 11. The apparatusaccording to claim 10, wherein a plurality of infrared radiator devicesare arranged in such a way that the tenterhooks are moved essentiallybelow said radiator devices.
 12. The apparatus according to claim 11,wherein the infrared radiator devices are arranged essentially parallelto the tenterhooks and above the film edge as well as above an upperlimit of a region covered by the tenterhook levers.
 13. The apparatusaccording to claim 7, wherein the hot-air heating device is provided, inheating zones, with a nozzle aligned essentially parallel to and inadvance of the film edge which is aligned essentially with the filmedge.
 14. The apparatus according to claim 13, wherein a hot-air duct ofthe hot-air heating device reaches over the infrared radiator devicesuch that an outlet nozzle of the hot-air heating device is offset inthe direction of the center of the plastic film with respect to theinfrared radiation region.
 15. The apparatus according to claim 7,wherein the infrared radiator devices radiate at a wavelength of 0.1 to10 μm.
 16. The apparatus according to claim 7, wherein, in addition tothe simultaneous heating of the film edge by means of infrared radiatorsand hot-air heating devices, an additional infrared radiator device orhot-air heating device is provided for the central portion of the film.17. The apparatus according to claim 10, further including a measuringdevice for measuring the film edge temperature comprising a plurality ofpyrometers aligned essentially with the film edge, and that the filmedge temperature is determined, via said pyrometers, taking into accounta mixed temperature which is obtained from the measurement, occurringduring the measuring cycle, including the tenterhook temperature and thefilm edge temperature in the space between two adjacent tenterhooks. 18.The apparatus according to claim 17, wherein the pyrometers are arrangedsuch that the space between tenterhooks moving past and the film edge isdetectable between two tenterhooks, can be carried out without anysignal fluctuation or essentially without any signal fluctuation. 19.The apparatus according to claim 17, wherein at least one furtherpyrometer arrangement is provided which measures only the tenterhooktemperature.
 20. The apparatus according to claim 19, wherein thepyrometer arrangement for measuring the tenterhook temperature isoriented with a tangential component to the film edge such that, in eachcase, at least one tenterhook is located in the detection range of saidpyrometer arrangement and the film or the film edge is essentially orcompletely shielded.
 21. The apparatus according to claim 17 wherein thefilm temperature (S_(F)), at which the film edge heating device isactivated is determined by means of an electronic evaluation and/orcontrol device from a measured signal for the tenterhook temperature anda mixed signal for the mixed temperature.